CTE or Subselect and outer joins not behaving as expected?

Hi all, I'm trying to enumerate a list of months between a date in the past and now and display a value from an existing table if there is one for the date or NULL if there isn't.

I'm using this SQL to generate the months:

select distinct date_trunc( 'month', '2011-06-01'::date + tally_day )::date as tally_mon
from generate_series( 0, ( select current_date - '2011-06-01'::date ) ) as tally_day

[[ produces ]]
tally_mon
------------
2011-06-01
2011-07-01
2011-08-01
2011-09-01
2011-10-01
(5 rows)

and I am trying to use it as the left hand side of a left join against my data table:

select tally_table. tally_mon, met.datum
from (
select distinct date_trunc( 'month', '2011-06-01'::date + tally_day )::date as tally_mon
from generate_series( 0, ( select current_date - '2011-06-01'::date ) ) as tally_day
) as tally_table full outer join my_existing_table as met on( tally_mon = met.month )
where met.series = 1;
-- ** See SETUP below **

This produces rows only for those that exist in my_existing_table and no left join output which I'd expect for the rest of the tally_table rows.

What am I missing?

Many thanks,
Joel

SETUP:
create temp table my_existing_table ( month date not null, series int not null, datum int not null );
insert into my_existing_table values ( '2011-08-01', 1, 5 ), ( '2011-10-01', 1, 4 );